In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as having a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an optical waveguide. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus, producing a guiding effect. The most frequently used material for such a waveguide device is glass, which is formed into a fiber of specified dimensions.
As the development of optical circuits proceeded, it became necessary to have devices which could couple, divide, switch and modulate the optical waves from one waveguide fiber to another.
Some optical fibers are interconnected by other optical fibers cut to length. These devices have only two terminals-one at each end. Photohardened films containing a waveguide have been proposed for this use, such as in U.S. Pat. No. 3,809,732. However, the device disclosed therein cannot be easily coupled to and aligned with an optical fiber. Further, due to the uneven surface of its film, one cannot easily protect its exposed surface from the environment.
Another method used to form an optical coupling device involves the application of standard photolithographic processes and diffusion. By this prior art process, standard lithographic processes are used to define a pattern in a photoresist layer deposited on a chosen substrate. Then, an etchant is applied to etch the photoresist-defined pattern into the substrate. Next, a metal is deposited in the etched region by vacuum deposition. The photoresist pattern is then lifted off with an appropriate solvent, carrying with it unwanted metal deposits. The structure is then heated to diffuse the metal deposited in the etched region into the substrate, to form a waveguiding layer therein. See, for instance, U.S. Pat. No. 4,609,252. In addition to the fact that many steps are involved in such a process, there is also a limitation on the thickness of the metal which may be deposited. First, since vacuum deposition is a relatively slow process, there is the limitation of the excessive amount of time required to deposit a thick layer of metal. Secondly, as more and more metal is deposited, new centers for deposition are created, resulting in an uneven deposit.
To form branches, two or more fibers have been bonded to a common optical port using an adhesive having an index of refraction closely matched to that of the fibers. The fibers are very small in diameter and must be handled with extreme care, bundled together for strength, and attached to a support at intervals. Fabrication of the equivalent of a printed circuit board comprised of these discrete fibers and optical devices is labor-intensive, expensive, slow, tedious, and not readily adapted to automated fabrication techniques. Another method used to form such a coupler is to fuse or melt fibers together so that light from one fiber can pass to the connected fibers. However, in such a fusion process it is difficult to control the extent of fusion and the exact geometry and reproducability of the final structure.
A device of particular interest is the "Y-coupler", which is a "y"-shaped device that couples signals together or divides them apart. "Y"-shaped devices have been made in a wet process by exposing a liquid photoactive layer to ultraviolet radiation through a mask. Then a solvent is used to remove the unpolymerized portions of the layer. See, for instance, U.S. Pat. No. 4,609,252. The waveguide of this device, like those mentioned above, isn't protected from the environment or readily coupled to an optical fiber. Further, being a wet process, it has the tendency of being messy and the problem of disposing of the spent solvent.
Another "Y"-shaped coupler device is disclosed in U.S. Pat. No. 4,666,236. It further discloses a device with one input branch and three output branches. These devices are also made by a wet process exposing a liquid photopolymer film to light to create a waveguide. The unexposed liquid film is dried and becomes part of the device. The film is further coated with a layer, such as an acrylic resin, to prevent deposition of dust and staining. Again, this process is wet and, thus, inherently messy.
U.S. Pat. No. 3,809,686 shows waveguides created in a single photopolymer film by focusing a beam of light within the film and moving the film. It shows multiple waveguides in a single film. In one embodiment, the waveguides exhibit evanescent coupling of light between the waveguides. It further teaches the creation and use of holographic diffraction gratings as light couplers. However, it is difficult to focus light within a film to form a homogenous waveguide with clear and distinct boundaries.
Superior utilization of optical communication networks may be achieved by wavelength division multiplexing. In this technique light of two or more different wavelengths is simultaneously transmitted through a single transmission channel, thus making use of the low-loss characteristics of optical fibers over a wide wavelength region. For the advantages of this technique to be realized, optical multiplexers, devices which combine light of different wavelengths into a single transmission channel, and demultiplexers, devices which separate light containing a mixture of wavelengths into individual light beams of different wavelengths, are required. Since two-way transmission, that is, transmission of information through the channel in both directions, requires a multiplexer and a demultiplexer at each end of the system, devices which can perform both functions are preferred in this application. Although the devices described in this application perform both functions, i.e., multiplexing and demultiplexing, they will be referred to as demultiplexers or demultiplexing devices, even though they could also be referred to as multiplexers or as multiplexers-demultiplexers.
As described in Seki et al. U.S. Pat. No. 4,790,615 demultiplexers for integrated-optic circuits are known. These devices comprise an optical waveguide formed by, e.g., ion exchange in a transparent substrate, e.g., glass. The waveguide is branched, comprising an input path, a branching region, a transmitting path, and an output path. A demultiplexing filter which passes (or reflects) light having a specific predetermined wavelength and reflects (or passes) light of other wavelengths is fitted into a groove machined into the glass substrate through the branching region. One such device, the Photocor.TM. integrated-optic wavelength division multiplexer sold by Corning Glass Works, is manufactured by thallium ion exchange to generate waveguides on a glass substrate followed by micromachining the glass substrate to produce narrow channels into which wavelength selective filters can be placed.
In the manufacture of these devices, the groove for the demultiplexing filter must be accurately machined into the glass substrate. If the angle of the groove with respect to the waveguide is not correct, loss at the branching region will increase. Consequently the manufacture of such devices is also labor-intensive, expensive, slow and tedious, and not readily suited to automated production techniques. A need exists for a demultiplexer suitable for use with integrated optic circuits which is not labor-intensive, expensive, and slow and tedious to manufacture; whose manufacture does not have all the disadvantages associated with a wet process; and which is suited to automated production techniques.